For example, a choke valve is provided in a small two-stroke engine, or the like. The choke valve is provided between a carburetor and an air cleaner. The choke valve increases a fuel proportion to intake air by being closed at the time of starting an engine. Meanwhile, there is a known phenomenon referred to as “blowing back” in which a fuel flows backward from the carburetor to the air cleaner during driving of the engine. When blowing back occurs, the fuel is attached to the air cleaner to cause fuel sagging from an air cleaner box, or air content decreases due to humidity in an air cleaner element to cause a decrease in engine power. A blowing back prevention plate is provided between the carburetor and the air cleaner in some cases to reduce a problem due to blowing back. A large amount of fuel may be prevented from being attached to the air cleaner by the provision of the blowing back prevention plate.
Meanwhile, there is a known technology for enhancing tightness by the choke valve. For example, as described in Japanese Examined Utility Model Publication No. H6-29489, there is a known structure in which a choke plate is provided to block an air intake passage, and the choke plate and a choke lever are attached to a cleaner case. A cleaner cover is attached to the cleaner case. In the structure described in Japanese Examined Utility Model Publication No. H6-29489, a projection is provided on the choke plate, and a rib is provided on a rear surface of the cleaner cover. The rib is formed in an arc shape corresponding to a trajectory of the projection on the choke plate. The rib forms a tapered shape in a height direction.